Endocytosis and Clathrin-Coated Vesicles

Endocytosis

• Cells take in materials via invagination of the plasma membrane.
• A small portion of the plasma membrane encloses the material, buds inward, and pinches off forming an intracellular endocytic vesicle.
• Endocytosis transports receptor-ligand complexes, nutrients and their carriers, extracellular matrix components, cell debris, bacteria, viruses, and even other cells.
• Endocytosis regulates the composition of the plasma membrane.

Clathrin and Clathrin-Coated Vesicles

• Clathrin forms a basket-like network on the cytosolic surface of the membrane.
• Clathrin coats are made of triskelions (three-legged structures).
• Clathrin-coated vesicles bud off from the plasma membrane and transport molecules into the cell.

Formation of Clathrin-Coated Vesicles

• Adaptor proteins (adaptins) help select cargo molecules for transport.
• Adaptins secure the clathrin coat to the membrane.
• Clathrin does not determine the specificity of the cargo.

Dynamin and Membrane Bending

• Dynamin is a GTPase required to pinch off the vesicle from the plasma membrane.
• Dynamin assembles into helical structures and constricts the membrane neck.
• Membrane bending proteins assist in the pinching off process.

Types of Endocytosis

• Pinocytosis is "cellular drinking" involving the ingestion of fluid and molecules via small pinocytic vesicles.

• Receptor-mediated endocytosis is a specific type of endocytosis.

• Phagocytosis is "cellular eating" where large particles, like bacteria or cell debris are engulfed.

Endocytosis

• The engulfment of extracellular material into the cell through the invagination of the plasma membrane.
• This process forms intracellular endocytic vesicles.
• Endocytosed cargo includes receptor-ligand complexes, nutrients, extracellular matrix components, cell debris, bacteria, and viruses.
• Cells can regulate the composition of their plasma membrane through endocytosis

Clathrin and Clathrin-Coated Vesicles

• Clathrin-coated vesicles are involved in the endocytosis of many molecules.
• The clathrin coat is a basket-like network on the cytosolic surface of a membrane.
• EM images
  • Reveal numerous clathrin-coated vesicles budding from the inner surface of the plasma membrane.
  • Show clathrin triskelions
• Formation of clathrin-coated vesicles
  • EM images depict various stages of clathrin-coated vesicle budding.

Adaptins

• Adaptor proteins that link cargo molecules to the clathrin coat.
• They secure the coat to the membrane.
• Clathrin itself does not directly select specific molecules for transport.

Dynamin

• A large GTPase that helps to pinch off a vesicle from the plasma membrane.
• Dynamin assembles into helical structures.
• GTP hydrolysis by dynamin causes conformational changes in the helix, constricting the membrane neck.
• Visualizing Dynamin
  • In purified dynamin visualized by EM, it forms helical structures around the neck of the vesicle.
  • Clathrin-RFP and Dynamin-GFP are used to visualize clathrin and dynamin dynamics at the plasma membrane.
  • Dynamin mutants that cannot hydrolyze GTP cannot pinch off the vesicle.

Types of Endocytosis

• Pinocytosis "cellular drinking" - ingestion of fluid and molecules via small pinocytic vesicles.
• Receptor-mediated endocytosis - specific molecules are engulfed with the help of receptors on the cell surface.
• Phagocytosis "cellular eating" - ingestion of large particles, such as bacteria or cellular debris by large phagosomes.

Autophagy

• A process where a cell degrades its own components by encapsulating them within a double membrane
• This process forms autophagosomes.

Endocytosis & Vesicular Transport

• The process of cells taking in materials through the invagination of the plasma membrane, forming an intracellular endocytic vesicle.
• Endocytosed cargo includes receptor-ligand complexes, nutrients, extracellular matrix components, debris, bacteria, viruses, and even other cells.
• Endocytosis regulates the composition of a cell's plasma membrane.

Clathrin-Coated Vesicles

• Clathrin forms a basket-like network on the cytosolic surface of the membrane, aiding in the formation of clathrin-coated vesicles.
• Clathrin triskelions are the building blocks of the clathrin coat.

Formation of Clathrin-Coated Vesicles

• Adaptins (adaptor proteins) help select cargo molecules for transport and secure the clathrin coat to the membrane.
• Clathrin itself does not play a role in cargo selection.

Dynamin

• Dynamin is a large GTPase that assembles into helical structures and is crucial for pinching off clathrin-coated vesicles from the plasma membrane.
• GTP hydrolysis by dynamin drives conformational changes that constrict the membrane neck.
• Membrane bending proteins assist dynamin in pinching off the vesicle.

Types of Endocytosis

• Pinocytosis: "Cellular drinking" involving the ingestion of fluid and molecules via small pinocytic vesicles.
• Receptor-mediated endocytosis: A highly specific process where receptors bind to specific cargo molecules, initiating their internalization into the cell.
• Phagocytosis: "Cellular eating" engulfing large particles like bacteria, cell debris, and other cells, forming phagosomes.

This quiz explores the process of endocytosis, focusing on the role of clathrin and clathrin-coated vesicles in transporting materials into cells. Understand how these vesicles form and the importance of adaptin proteins in the selection of cargo molecules. Test your knowledge on cellular transport mechanisms.